1. Field of the Invention
This invention is directed to apparatus for molding polymer foam and is more particularly directed to apparatus for fabricating polymer foam buns from isocyanate-based polymer foam forming mixtures.
2. Description of the Prior Art
Processes for the preparation of substantially isotropic polymer foams derived from isocyanates in the form of continuous bunstock are well-known in the art; see, for example, U.S. Pat. Nos. 3,249,486; 3,655,311 and 3,734,668. Said processes require the use of complex machinery and ancillary storage facilities for polymer foam chemicals, all of which are not readily transportable. Accordingly, it is customary to manufacture foam bunstock at a central production facility and to transport the finished bun to the site of use by any appropriate means. Due to the high cost of transporting such buns by commercial freight, the area which can be served from a central manufacturing plant is relatively limited. Since the polymer chemicals employed in the manufacture of the bun occupy approximately 30 times less space than the finished foam, it is theoretically more economical to transport the chemicals to the site at which the foam is to be used and to manufacture the foam on the site. The sheer bulk of the continuous bunstock manufacturing plant clearly makes it impossible to transport the latter to the site at which the foam is to be used. An even greater problem exists where the foam is to be fabricated in relatively undeveloped countries where, even if the requisite machinery for manufacture of continuous bunstock is available, the means of transporting foam from the manufacturing site to the site of its use may either be unavailable or unsatisfactory.
It has therefore long been desirable to develop a satisfactory method of molding polymer foam buns which employs relatively simple and readily transportable molding means which can be brought to, and used at, the site at which the foam is to be used. It is an object of this invention to provide a molding apparatus which meets these requirements. It is also an object of this invention to provide molding apparatus for the preparation of relatively low density (of the order of 2 to 4 pcf.), substantially isotropic, isocyanate-based rigid polymer foams. By isotropic foam is meant a rigid foam which has uniform cell structure and which exhibits substantially equal strength characteristics along the X, Y and Z direction axes to which a compressive force is applied.
The use of a box mold having fixed sides and top is not suitable for the preparation of isotropic rigid isocyanate-based polymer foams. The use of molds having "floating" lids has been described; see, for example, Belgian Pat. No. 824,625 and U.S. Pat. No. 3,291,873. The use of such molds gives foams which have improved cell structure in the upper layer but still does not produce useful isotropic foam.
In the preparation of continuous bunstock described in the aforesaid U.S. Pat. No. 3,249,486, the mold is provided with a fixed upper mold piece and sidewalls which are pivoted at their upper edges and are free to be pushed outwardly about the pivoted edge against the action of retaining springs. Such a procedure does not give rise to truly isotropic foam and, even if it did, is not readily adaptable to use in a box mold for batch type production.